High temperature precision furnaces for the heat processing of products and materials often employ a furnace muffle formed of quartz which can withstand the extremely high operating temperatures. In a multi-zone furnace in which different temperatures and/or gases can be maintained, a barrier is provided between adjacent furnace sections to maintain distinct gas environments in the adjacent zones or to alternatively or additionally maintain a predetermined temperature profile between adjacent zones. The barrier is constructed of metal and must be sealed to the quartz muffle to prevent gas leakage from the muffle interior, which leakage can adversely affect the composition of the environment to be maintained within the muffle. In addition, in the case of toxic, volatile or explosive gases, leakage from the muffle can lead to disasterous consequences. The sealing of the quartz muffle to the metal barrier cannot be easily accomplished by reason of the relatively large difference in the thermal coefficients of expansion of the materials. In addition, known glass to metal sealing techniques are not practical by reason of the relatively large area to be sealed and the extremely high operating temperatures at which the seal must remain intact. In furnaces of known construction, seals known as cold seals such as spring loaded rubber gaskets have been provided in housings outside of the muffle but such cold seals cannot be employed at high operating tempertures, say in the vincinty of 600.degree. C.